Fuel cell systems include a fuel cell stack that produces electrical energy based on a reaction between a hydrogen-based feed gas (e.g., pure hydrogen or a hydrogen reformate) and an oxidant feed gas (e.g., pure oxygen or oxygen-containing air). The hydrogen-based feed gas and oxidant feed gas are supplied to the fuel cell stack at appropriate operating conditions (i.e., temperature and pressure) for reacting therein. The proper conditioning of the feed gases is achieved by other components of the fuel cell system to provide the proper operating conditions.
The fuel cell stack includes multiple fuel cells electrically connected in series. Each fuel cell includes a polymer electrolyte membrane (PEM) sandwiched between a cathode plate and an anode plate. Electrically conductive diffusion media are disposed between the PEM and both the cathode and anode plates. The cathode plate includes cathode flow channels, through which the oxidant feed gas flows. Similarly, the anode plate includes anode flow channels, though which the hydrogen feed gas flows. The cathode and anode flow channels are open to the diffusion media to enable diffusion of the oxidant and hydrogen feed gases to the PEM.
As the oxidant stream travels through the fluid flow channels of the reactant plates, the stream absorbs water that is produced as the product of the electrochemical reaction. The product water is absorbed either as water vapor or as entrained water droplets. As a result, an initial portion of the flow field is dryer than a latter portion (e.g., just prior to being exhausted from the fuel cell). In the latter portion, the oxidant stream can become saturated with water and two phase flow occurs. More specifically, the oxidant stream contains water vapor and liquid water entrained in the oxidant stream.
Wet and dry regions of the flow field can detrimentally affect fuel cell performance and accelerate the degradation of performance over time. Fuel cell performance is defined as the voltage output from the cell for a given current density. Control of water transport through the cathode-side diffusion media to the oxidant flow channels is important to optimizing fuel cell performance.